The technical field relates to microfabrication to form electrical conductors, and, in particular, to electrodeposited photoresist patterning.
Photolithography and photomasking to generate a device pattern on a wafer substrate are an essential part of fabrication of electrical conductor devices. A photolithographic device fabrication process involves covering regions of a thin film on a silicon (Si) wafer substrate that are not to be etched with a photoresist mask, and etching the thin film from regions that are not protected by the photoresist mask. As a result, conductor traces may be produced on a fabrication surface. The fabrication surface of the Si wafer is usually flat, and layers of the thin films are applied and removed on top of the flat fabrication surface. A more complicated geometry arises if the wafer itself is etched in regions that are photodefined, so that trenches or recesses whose depths are dependent on the duration of the etching, or through-holes or slots that go all the way through the wafer, may be produced on the fabrication surface. The substrate then is not planar but rather is a three-dimensional surface, upon which thin films conductors can be applied and patterned. In some applications, electrical conductors need to be patterned on top of a dielectric film on a sloped surface of a Si wafer substrate or die. The sloped surface may be either on an edge of the wafer substrate or in slots or trenches within the wafer substrate.
To pattern the electrical conductor traces on the Si wafer substrate, photoresist masks must be applied uniformly to the sloped surface of the wafer substrate. When the fabrication surface lacks flatness or includes a sloped surface, it is difficult to produce a photoresist film of high uniformity and to produce a precise exposure over the entire surface. To pattern the electrical conductor traces on the Si wafer substrate, photoresist masks must be applied uniformly to the sloped surface of the wafer substrate. Spin coating is for non-planar substrates is described, for example, in xe2x80x9cNew Photoresist Coating Method for 3-D Structured Wafers,xe2x80x9d Sensors and Actuators 85, 2000, by Kutchoukov et al. Kutchoukov et al. present a method for the uniform coating with standard photoresist of a (100)-Si semiconductor wafer containing anisotropically wetetched through-holes. The coating method is based on photoresist dispensing on the mask opening side of the wafer and spinning the wafer from both sides in a saturated photoresist solvent atmosphere.
Another method for applying photoresist over complex three-dimensional surfaces is by electroplating. Thereafter, the photoresist is exposed to ultraviolet (UV) light in a desired pattern and selectively removed to define the pattern so that the metal conductors traces may be formed.
Patterning the photoresist after the photoresist is applied onto a three-dimensional sloped substrate is a further complication. A UV light source typically projects a mask, which contains the desired pattern, onto the substrate. An excimer laser is sometimes used as the UV source for exposing the photoresist to the mask pattern. U.S. Pat. No. 4,773,750, entitled xe2x80x9cDeep-UV Lithography,xe2x80x9d describes using an excimer laser for lithography, where fine features can be made on a flat surface using a projection lens. However, since photo imaging tools, such as lithography machines or projection lenses, are designed to make precise features on the wafer surface, the photo imaging tools typically have limited depth of focus. Therefore, a patterned mask does not work well on a sloped surface because the pattern typically loses definition on the slope.
FIG. 1 illustrates the problem with using a projection lens for patterning photoresist on a sloped surface. A Si wafer substrate 110 has a sloped surface 160(c). In order to pattern conductor traces 170 on the sloped surface 160(c), two projection lenses 180(a), 180(b) are exposed onto the sloped surface 160(c). Due to the depth of the sloped surface 160(c) and the limited focus of the projection lenses 180(a), 180(b), the projection lens 180(b) directed at a trench 160(b) of the sloped surface 160(c) tends to lose definition. As a result, the photoresist pattern typically becomes out of focus as the sloped surface 160(c) moves away from a surface 160(a) of the wafer substrate 110.
A method for using an excimer laser to pattern electrodeposited photoresist on a sloped surface includes depositing a layer of photoresist on top of a substrate that includes a sloped surface and scanning an excimer laser beam over the layer of photoresist to expose the layer of photoresist in a desired pattern. The scanning step includes projecting the excimer laser beam in a small beam spot onto the substrate and scanning the small beam spot of the excimer layer beam relative to the substrate to define the pattern sequentially onto the substrate, including the sloped surface.
One embodiment of an apparatus for using an excimer laser to pattern electrodeposited photoresist on a sloped surface of a substrate includes an excimer laser beam scanned over a layer of photoresist to expose the layer of photoresist in a desired pattern, a mask with an aperture that selects a portion of the excimer laser beam to be projected onto the substrate, and a projection lens that projects an image of the aperture as a small beam spot onto the substrate. The pattern may be sequentially defined onto the substrate, including the sloped surface, by moving the small beam spot of the excimer layer beam relative to the substrate.
Another embodiment of the apparatus for using an excimer laser to pattern electrodeposited photoresist on a sloped surface of a substrate includes an excimer laser beam scanned over a layer of photoresist to expose the layer of photoresist in a desired pattern and a telescope lens system that forms a collimated excimer laser beam to be projected onto the substrate. The pattern may be sequentially defined onto the substrate, including the sloped surface, by moving the small beam spot of the excimer layer beam relative to the substrate.
Since the small beam spot of the excimer laser beam does not diverge substantially, no refocusing or repositioning is necessary to pattern the layer of photoresist on the sloped surface. Accordingly, tight tolerances and fine lines and spaces of the pattern may be maintained, eliminating the need for a complicated processing sequence.